MPLS MTU Command Changes

This document explains the change in the behavior of the mpls mtu command for the following Cisco IOS releases:

•12.2(27)SBC and later

•12.2(33)SRA and later

•12.2(33)SXH and later

•12.4(11)T and later

•15.0(1)M1

•15.1(2)S

You cannot set the Multiprotocol Label Switching (MPLS) maximum transmission unit (MTU) to a value larger than the interface MTU value. This eliminates problems such as dropped packets, data corruption, and high CPU rates from occurring when the MPLS MTU value settings are larger than the interface MTU values. Cisco IOS software allows the MPLS MTU value to be higher than the interface MTU value only for interfaces that have a default interface MTU value of 1580 or less.

Note In Cisco IOS Release 15.0(1)M1, you can configure the interface MTU on PA-1FE and PA-2FE port adapters (PAs). The range of values is from 1500 to 1530. Before this enhancement, the MTU of those interfaces was not configurable, and any attempt to configure the interface MTU displayed the following message:

% Interface {Interface Name} does not support user settable mtu.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for MPLS MTU Command Changes" section.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn.An account on Cisco.com is not required.

Information About MPLS MTU Command Changes

MPLS MTU Values During Upgrade

If you have configuration files with MPLS MTU values that are larger than the interface MTU values and you upgrade to Cisco IOS Release 12.2(27)SBC, 12.2(33)SRA, 12.4(11)T, 12.2(33)SXH, or later releases, the software does not change the MPLS MTU value. When you reboot the router, the software accepts the values that are set for the MPLS MTU and the interface MTU. The following error message is displayed during system initialization:

Setting the mpls mtu to xxxx on interface x/x, which is higher than the interface MTU
xxxx. This could lead to packet forwarding problems including packet drops.

You must set the MPLS MTU values equal to or lower than the interface MTU values.

Caution If you do not set the MPLS MTU less than or equal to the interface MTU, data corruption, dropped packets, and high CPU conditions can occur.

Guidelines for Setting MPLS MTU and Interface MTU Values

When configuring the network to use MPLS, set the core-facing interface MTU values greater than the edge-facing interface MTU values using one of the following methods:

•Set the interface MTU values on the core-facing interfaces to a higher value than the interface MTU values on the customer-facing interfaces to accommodate any packet labels, such as MPLS labels, that an interface might encounter. Make sure that the interface MTUs on the remote end interfaces have the same interface MTU values. The interface MTU values on both ends of the link must match.

•Set the interface MTU values on the customer-facing interfaces to a lower value than the interface MTU on the core-facing interfaces to accommodate any packet labels, such as MPLS labels, that an interface might encounter. When you set the interface MTU on the edge interfaces, ensure that the interface MTUs on the remote end interfaces have the same values. The interface MTU values on both ends of the link must match.

Changing the interface MTU can also modify the IP MTU, Connectionless Network Service (CLNS) MTU, and other MTU values because they depend on the value of the interface MTU. The Open Shortest Path First (OSPF) routing protocol requires that the IP MTU values match on both ends of the link. Similarly, the Intermediate System-to-Intermediate System (IS-IS) routing protocol requires that the CLNS MTU values match on both ends of the link. If the values on both ends of the link do not match, IS-IS or OSPF cannot complete initialization.

If the configuration of the adjacent router does not include the mpls mtu and mtu commands, add these commands to the router.

Note The MPLS MTU setting is displayed only in the show running-config output if the MPLS MTU value is different from the interface MTU value. If the values match, only the interface MTU value is displayed.

If you attempt to set the MPLS MTU value higher than the interface MTU value, the software displays the following error message, which prompts you to set the interface MTU to a higher value before you set the MPLS MTU value:

% Please increase interface mtu to xxxx and then set mpls mtu

Note In Cisco IOS Release 15.1(2)S, the mpls mtu command was modified. This command was made available in L3VPN encapsulation configuration mode. The maximum keyword was replaced with the max keyword. The override keyword and the bytes argument were removed from the GRE tunnel interface. To set MPLS MTU to the maximum MTU on L3VPN profiles, use the mpls mtu command in L3VPN encapsulation configuration mode.

MPLS MTU Values for Ethernet Interfaces

If you have an interface with a default interface MTU value of 1580 or less (such as an Ethernet interface), the mpls mtu command provides an overridekeyword, which allows you to set the MPLS MTU to a value higher than the interface MTU value. The override keyword is not available for interface types that do not have a default interface MTU value of 1580 or less. For configuration details, see the "Setting the MPLS MTU Value on an Ethernet Interface" section.

Setting the MPLS MTU value higher than the Ethernet interface MTU value can lead to dropped packets, data corruption, or high CPU rates. When you set the MPLS MTU value higher than the Ethernet interface MTU value, the software displays the following message:

%MFI-30-MPLS_MTU_SET: Setting the mpls mtu to xxxx on Ethernet x/x, which is higher than
the interface MTU xxxx. This could lead to packet forwarding problems including packet
drops.

Most drivers will be able to support baby giants and will gracefully drop packets that are
too large. Certain drivers will have packet forwarding problems including data corruption.

Setting the mpls mtu higher than the interface mtu can lead to packet forwarding problems
and may be blocked in a future release.

Note Theoverride keyword is supported in Cisco IOS Release 12.2(27)SBC, 12.2(33)SRA, 12.4(11)T, 12.2(33)SXH, and later releases, but may not be supported in a future release.

How to Configure MPLS MTU Values

The following sections explain how to configure MPLS MTU and interface MTU values:

Setting the Interface MTU and MPLS MTU Values

Use the following steps to set the interface MTU and the MPLS MTU.

Note In Cisco IOS Release 15.0(1)M1, you can configure the interface MTU on PA-1FE and PA-2FE port adapters. The range of values is from 1500 to 1530. Before this enhancement, the MTU of those interfaces was not configurable.

Example: Setting the Interface MTU and MPLS MTU

The following example shows how to set the interface and MPLS MTU values. The serial interface in the following configuration examples is at the default interface MTU value. The MPLS MTU value is not set. The interface settings are as follows:

interface Serial 4/0

ip unnumbered Loopback0

mpls traffic-eng tunnels

mpls ip

serial restart-delay 0

ip rsvp bandwidth 2000 2000

end

The following example attempts to set the MPLS MTU value to 1520. This returns an error because MPLS MTU cannot be set to a value greater than the value of the interface MTU.

Router# configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Router(config)# interface serial 4/0

Router(config-if)# mpls mtu 1520

% Please increase interface mtu to 1520 and then set mpls mtu

The following example first sets the interface MTU to 1520 and then sets the MPLS MTU to 1520:

Router(config-if)# mtu 1520

Router(config-if)# mpls mtu 1520

The following example shows the new interface MTU value. The MPLS MTU value is not displayed because it is equal to the interface value.

Router#show running-config interface serial 4/0

Building configuration...

interface Serial4/0

mtu 1520

ip unnumbered Loopback0

mpls traffic-eng tunnels

mpls ip

serial restart-delay 0

ip rsvp bandwidth 2000 2000

end

The following example sets the MPLS MTU value to 1510:

Router(config-if)# mpls mtu 1510

The following example shows the new interface MTU value. The MPLS MTU value is displayed because it is different than the interface MTU value.

Router#show running-config interface serial 4/0

Building configuration...

interface Serial4/0

mtu 1520

ip unnumbered Loopback0

mpls mtu 1510

mpls traffic-eng tunnels

mpls ip

serial restart-delay 0

ip rsvp bandwidth 2000 2000

end

Example: Setting the MPLS MTU Value on an Ethernet Interface

Caution Setting the MPLS MTU to a value higher than the Ethernet interface MTU value can lead to dropped packets, data corruption, or high CPU rates.

The following example shows how to set the MPLS MTU values on an Ethernet interface. The Ethernet interface in the following configuration examples is at the default interface MTU value. The MPLS MTU value is not set. The interface settings are as follows:

interface Ethernet 2/0

ip unnumbered Loopback0

mpls traffic-eng tunnels

mpls ip

serial restart-delay 0

ip rsvp bandwidth 2000 2000

end

The following example uses the override keyword to set the MPLS MTU to 1520, which is higher than the Ethernet interface's MTU value:

Router(config-if)# mpls mtu override 1520

%MFI-30-MPLS_MTU_SET: Setting the mpls mtu to 1520 on Ethernet2/0, which is higher than
the interface MTU 1500. This could lead to packet forwarding problems including packet
drops.

The following example shows the new MPLS MTU value:

Router#show running-config interface ethernet 2/0

Building configuration...

interface Ethernet 2/0

mtu 1500

ip unnumbered Loopback0

mpls mtu 1520

mpls traffic-eng tunnels

mpls ip

serial restart-delay 0

ip rsvp bandwidth 2000 2000

end

Example: Setting the MPLS MTU Value to the Maximum MTU on L3VPN profiles

The following example shows how to set the MPLS MTU value to the maximum MTU on L3VPN profiles:

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Feature Information for MPLS MTU Command Changes

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Note Table 1 lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

This document explains the changes to the mpls mtu command. You cannot set the MPLS MTU value larger than the interface MTU value, except for Ethernet interfaces.

In 12.2(28)SB, support was added for the Cisco 10000 router.

In 12.2(33)SRA, support was added for the Cisco 7600 series router.

This feature was integrated intoCisco IOS Release 12.4(11)T.

This feature was integrated intoCisco IOS Release 12.2(33)SXH.

In 15.0(1)M1, you can configure the interface MTU on PA-1FE and PA-2FE port adapters.

In 15.1(2)S, the mpls mtu command was made available in L3VPN encapsulation configuration mode. The maximum keyword was replaced with the max keyword. The override keyword and the bytes argument were removed from the GRE tunnel interface.

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